mtest: make `meson test --list` return whether there was any test to list
mtest: make `meson test --list` return whether there was any test to list

becm/meson,becm/meson,jpakkane/meson,jpakkane/meson,pexip/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,becm/meson,jpakkane/meson,pexip/meson,QuLogic/meson,pexip/meson,QuLogic/meson,pexip/meson,QuLogic/meson,becm/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,becm/meson,pexip/meson,QuLogic/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,becm/meson,becm/meson,QuLogic/meson,becm/meson,becm/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,QuLogic/meson,mesonbuild/meson